The present invention relates to a method for fabricating a zinc oxide varistor for high voltage which exhibits a high breakdown voltage while at the same time having a good voltage-current characteristics by preventing the grain from becoming coarse according to liquid phase sintering methods used to fabricate a sintered body.
A zinc oxide varistor is a resistor having a non-linear voltage-current relationship. A zinc oxide varistor is a sintered body having zinc oxide as its major ingredient and containing bismuth oxide and manganese dioxide as additives. Such varistor is widely used as protecting elements of various kinds to protect electric and electronic instruments from lighting and switching surges. Varistors are also used as gapless type arrester elements for protecting the electric power system from falling of thunderbolt based upon their non-linear voltage-current characteristics and their excess voltage absorbing capacity.
The operating voltage of a zinc oxide varistor is determined in proportion to the number of potential barriers formed between both electrodes of the varistor.
Thus, the number of potential barriers per unit thickness becomes inversely proportional to the grain size, and the grain size becomes an important factor in determining the operating voltage.
According to a conventional method of fabricating a varistor (Jpn.: J. Appl. Phys. 10(6): 736-746 (1971), a varistor is fabricated by mixing very small amounts of bismuth oxide (Bi.sub.2 O.sub.3), cobalt oxide (Co.sub.3 O.sub.4), manganese dioxide (MnO.sub.2), antimony oxide (Sb.sub.2 O.sub.3), chromium oxide (Cr.sub.2 O.sub.3), and the like, and then sintered at a temperature of 1200.degree. C.-1350.degree. C. The bismuth oxide is melted during the sintering process and causes liquid phase sintering. Therefore, the zinc oxide grains by this fabrication process grow over 10 um.
Under these conditions the fabrication of a varistor having a breakdown voltage (V 1mA/mm) of over 300 volts becomes difficult. In addition to the above, the non-linear voltage-current characteristics and the uniformity of the characteristics are reduced because of the volatilization of bismuth oxide and the abnormal grain growth by the melted bismuth oxide liquid phase that occurs during the process.